That flatback plate was the same for most all the crossflow years, with the exception of the drilled hole for the telltale and the 82 and up plates that were drilled for the new-style trim control box. Regardless of your year, you could easily find a good used outer exhaust manifold and repl the one on your engine. I wouldn't worry about the year, just get a used one with the hose barb installed. Your engine is ancient, as it still has some of the original factory coils on it.